Are there kinematic equations that are not always true?

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  • #1
Lucho G
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Homework Statement:
Are there kinematic equations that are not always true? If so, which ones and in what cases?
Relevant Equations:
Cinematica
please I need to clarify this question
thanks sincerely
Luis
 

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  • #2
What do you mean by "true"? There are many equations which have a realm of validity, i.e. that rely on specific assumptions, if that's what you're asking.
 
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  • #3
Lucho G
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What do you mean by "true"? There are many equations which have a realm of validity, i.e. that rely on specific assumptions, if that's what you're asking.
i wanted to say the equations that are not always fulfilled in the kinematic.
 
  • #4
haruspex
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i wanted to say the equations that are not always fulfilled in the kinematic.
Any equation is only valid in specified circumstances. Even then, some are only approximations valid over a certain range.
You will need to be more specific about which equations concern you.
 
  • #5
Steve4Physics
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Homework Statement:: Are there kinematic equations that are not always true? If so, which ones and in what cases?
Let's assume :
1) by 'kinematics equations' you mean the elementary equations of constant acceleration, e.g. https://www.onlinemathlearning.com/image-files/suvat-equations.png
2) by 'true' you really mean 'applicable' or 'accurate'.

The equations are typically used for motion in a straight line with constant acceleration.

The equations are not applicable if the acceleration is not constant. A simple example is an object fallng through the air when air resistance is present. Can you explain why the acceleration is not constant in this case? (Hint, air resistance increases as speed increases).
 
  • #6
kuruman
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Homework Statement:: Are there kinematic equations that are not always true? If so, which ones and in what cases?
Relevant Equations:: Cinematica

please I need to clarify this question
thanks sincerely
Luis
Here is one. Distance = Speed × Time is not true in the case when the object accelerates. Can you think of another one? You have already received some hints.
 
  • #7
maxwells_demon
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the most prominent i can think about is the "max range" projectile equations. all of them assume 45 degrees as the optimal angle for launch. however that may not be true if the launch level and the landing level are different. which means all the shortcut equations related to this special case "burn and die".
 
  • #8
kuruman
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the most prominent i can think about is the "max range" projectile equations. all of them assume 45 degrees as the optimal angle for launch. however that may not be true if the launch level and the landing level are different. which means all the shortcut equations related to this special case "burn and die".
You get maximum range in projectile motion regardless of difference between launch and landing level if the initial velocity and the final velocity vectors are perpendicular. The 45o projection angle is a special case of that when the vertical velocity component just changes sign upon landing. You can find the details here.
 
  • #9
PhDeezNutz
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All the kinematics equations you are likely to be exposed to at the freshmen and sophomore level assume constant acceleration.

Throw constant acceleration out the window and the formulas are no longer valid (Obviously).
 
  • #10
kuruman
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All the kinematics equations you are likely to be exposed to at the freshmen and sophomore level assume constant acceleration.

Throw constant acceleration out the window and the formulas are no longer valid (Obviously).
All? Counter-example: Simple harmonic motion.
 
  • #11
maxwells_demon
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You get maximum range in projectile motion regardless of difference between launch and landing level if the initial velocity and the final velocity vectors are perpendicular. The 45o projection angle is a special case of that when the vertical velocity component just changes sign upon landing. You can find the details here.

oh yeah. mb. a faulty inference.

well, for what it's worth, all kinematic equations assume negligible air drag, and frictionless surfaces. all of them go out the window when we account for just how objects act irl.
 
  • #12
haruspex
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All the kinematics equations you are likely to be exposed to at the freshmen and sophomore level assume constant acceleration.
Specifically, the SUVAT equations.
But I see it as wrong to say the equations are not always 'true'. As I posted, all equations come with specifications of what the variables mean and under what conditions they apply; it's a package. The SUVAT equations are always true, but they can be misused.
For SUVAT it's not just that acceleration is constant, you also need to ensure displacement, velocity and acceleration are all being measured in the same straight line.
 
  • #13
PhDeezNutz
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All? Counter-example: Simple harmonic motion.

fair point.

Specifically, the SUVAT equations.
But I see it as wrong to say the equations are not always 'true'. As I posted, all equations come with specifications of what the variables mean and under what conditions they apply; it's a package. The SUVAT equations are always true, but they can be misused.
For SUVAT it's not just that acceleration is constant, you also need to ensure displacement, velocity and acceleration are all being measured in the same straight line.

I agree. Every equation (Or at least most that I can think of) and their assumptions are a package deal.
 

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